The present invention relates to methods and apparatus for monitoring respiration to determine whether there is breathing obstruction and for titrating nasal continuous positive airway pressure based thereon.
Obstructive sleep apnea syndrome (OSAS) is a well recognized disorder which may affect as many as 5% of the adult population. OSAS is one of the most common causes of excessive daytime somnolence. OSAS is most frequent in obese males, and it is the single most frequent reason for referral to sleep disorder clinics. OSAS is associated with all conditions in which there is anatomic or functional narrowing of the patient""s upper airway occurring during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea) to significant obstruction with or without reduced airflow (hypopnea and snoring), despite continued respiratory efforts. The morbidity of the syndrome arises from hypoxemia, hypercapnia, bradycardia, and sleep disruption associated with the apneas and arousal from sleep.
While monitoring respiration, it is frequently necessary to determine whether there is obstruction of breathing, whether the obstruction be manifested as apnea, hypopnea, or simply periods of high resistance which need not be accompanied by frank reduction in airflow. Identification of these events is used in both diagnosis of sleep disordered breathing and in the feedback control of automatically adjusting nasal CPAP therapy for obstructive sleep apnea syndrome. Whereas apnea and hypopnea are defined by absolute airflow, the recognition of partial obstruction can only be determined by calculating resistance. However, this generally requires invasive measurement of flow and respiratory effort, e.g., by intraesophageal pressure monitoring.
During therapeutic titration of nasal continuous positive airway pressure (CPAP) in patients with obstructive sleep apnea syndrome, residual apneas may occur that can be either obstructive or xe2x80x9ccentral.xe2x80x9d Differentiating these two types of apnea, which have different physiologic as well as therapeutic implications, may impact on the adjustments made to the CPAP pressure. On the one hand, if events are obstructive it is generally assumed that a higher pressure is needed. In contrast, if the events are central, the optimal response is not clearly defined at the present time, as these apneas may be transient irregularities of breathing such as those that occur after arousal (Marrone et al (1991)) or during REM. Not increasing the pressure in the presence of these central events has been recommended as desirable as part of the titration protocol (Teschler et al (1996); Series et al (1997)). In addition, clinical experience suggests that central apneas may even occur as a reaction to excessive CPAP (Berthon-Jones et al (1996); Boudewyns et al (1998)), and this suggests the need to lower the therapeutic pressure. Alternatively, some central apneas may respond to further increases in CPAP (Issa et al (1996)). Irrespective of the decision to raise, lower or maintain the CPAP when central apnea is detected, this decision can only be made, and the impact of the decisions tested, if the apneas are correctly classified.
In addition, during treatments of other breathing disorders in sleep (hypoventilation syndromes), positive pressure is applied to the airway in such a way as to both ventilate a patient (high pressure during inspiration) and maintain the airway free of obstruction, i.e., maintain patency (low pressure during expiration). Such bilevel devices are available, for example, under the trade names BiPap (Respironics), VPAP (Resmed), and MALLINKRODT 335 (Mallinkrodt). Adjustment of this lower expiratory pressure may be dictated by decisions similar to those dictated by setting CPAP in OSAS.
By definition, both types of apnea are identified by the absence of airflow. Differentiation between them is based on analysis of respiratory effort during the apneic period. This can be done either by non-invasive methods (e.g., impedance band) or from direct but invasive measurement of intrathoracic effort (e.g., esophageal balloon). Both of these approaches rely on more than the detection of airflow alone.
Prior methods for monitoring respiration are based on a mathematical analysis technique applied to the flow signal alone during nasal CPAP therapy. This technique is illustrated in U.S. Pat. Nos. 5,335,654, 5,490,502 and 5,803,066, the entire contents of each of which being hereby incorporated by reference. Thus, it has been demonstrated that one can recognize a surrogate of high resistance in the shape of the inspiratory airflow alone. This shape is known as a xe2x80x9cflow limitation contourxe2x80x9d and is a characteristic flattened contour seen on the inspiratory airflow curve. An example is shown in the top curve of FIG. 8 of U.S. Pat. No. 5,803,066, in the section marked xe2x80x9cflow limitationxe2x80x9d. It correlates highly with an elevated resistance and can be used in applications which rely on detecting abnormal behavior of the upper airway. Prior methods were based on recognition of this contour as a feedback variable for adjusting CPAP therapy.
Whereas clearly abnormal (flattened) and normal (sinusoidal) contours are readily identified, a significant number of breaths occur which are of ambiguous contour. In some individuals, these are merely variants of normal breath shapes and are not associated with partial obstruction. In other individuals, these intermediate shaped breaths are the only indication available that there is an abnormal resistance which results in clinical consequences (sleep disruption) and which requires treatment (e.g., raising the level of therapeutic CPAP). Misclassifying these breaths as to their resistance in either direction impedes optimal adjustment of CPAP therapy by either manual or automatic means.
A frequent incidental finding seen during monitoring of respiratory signals is the presence of cardiogenic oscillations (West et al (1961). These have been observed and reported during expiration as well as during apnea. Visible oscillations on the airflow signal during quiet exhalation are frequently seen during measurements made of pulmonary physiology, e.g., single breath nitrogen (Lauzon et al (1998) and diffusion studies (Brenner et al (1995). Detection of small movements at the cardiac frequency on inductive plethysmography or expired carbon dioxide signal (Kryger et al (1994)) during apnea has been suggested as an index of their xe2x80x9ccentralxe2x80x9d nature. More recently, similar oscillations have been observed on the airflow signal in adults and neonates during central apneas (Morrell et al (1995); Lemke et al (1996); Lemke et al (1998); Milner et al (1990); Shepard (1991)). Whereas Lemke et al. suggested that the presence of cardiogenic oscillations always correlated with a directly visualized open airway, Morrell et al. showed that similar oscillations were observed during central apneas regardless of the airway patency. Thus, there is no consensus on whether the presence of cardiogenic oscillations transmitted to the flow signal is dependent on patency of the airway, which can be compromised during the course of a xe2x80x9ccentralxe2x80x9d event, or on lack of respiratory effort.
In U.S. Pat. No. 5,803,066, a technique was identified to classify periods of apnea as being either obstructive or central. That patent discloses that if cardiac frequency pulsations (cardiogenic oscillations) can be detected on the airflow signal during such a period of apnea, the apnea is always classified as central. This implies that the apnea is never obstructive if cardiogenic oscillations are present.
The present invention provides a method for optimizing the controlled positive pressure in treating sleep disordered breathing by using the appearance or disappearance of cardiogenic oscillation in the airway signal as an additional parameter useful for classifying the level of resistance.
The present invention also provides a method for resolving an ambiguity in breath monitoring to determine whether or not breathing is labored due to an obstruction, by the presence or absence of cardiogenic oscillations.
According to the present invention, identification of inspiratory flow limitation can be accomplished with improved accuracy even when the breaths show a shape which is intermediate between definitely abnormal shape (flow limited) and definitely normal sinusoidal shape. This is accomplished by further examining whether there is cardiogenic oscillation present during expiratory periods and between breaths. This is detected by signal processing to enhance and identify small oscillations in the flow signal in the range of the pulse frequency in the range of the pulse, which oscillations represent cardiogenic oscillations. When these oscillations are detected, and breath whose shape is possibly abnormal, i.e., ambiguous, the breath may be classified as normal. When oscillation is absent, the threshold of the parameters used to classify the the shape of the inspiratory airflow abnormal is lowered and the breath is classified as having a high resistance. This technique is used to make the decision as to whether therapeutic CPAP pressure needs to be raised for obstructive events. It has the benefit of avoiding false positive detection of abnormally shaped breaths causing excessive rise in pressure in those patients who have them, while not sacrificing sensitivity to abnormal events in those who are more classical.
The inspiratory flow signal, both amplitude and contour, and the presence or absence of cardiogenic oscillations on the flow signal are used to define the state of resistance of the upper airway. Once the extent of obstruction is determined, appropriate adjustments are made to the CPAP pressure.
The present invention is an improvement on the method described in U.S. Pat. No. 5,803,066, by resolving ambiguities in the air flow path shape analysis. If the air flow pattern is ambiguous, and there are cardiogenic oscillations in the inter-breath period, then the breath is classified as having no obstruction, and treatment is provided based upon no obstruction. However, if the air flow pattern is ambiguous, and there are no cardiogenic oscillations, then the breath is classified as having an obstruction and treatment is provided based upon the presence of an obstruction.
The present invention may also be used to resolve ambiguities in other methods of determining the presence of an obstruction, such as use of the snoring method. If there is an ambiguous snore, which might be caused, for example, by other noise in the room, etc., the presence of oscillations would cause one to resolve the ambiguity against obstruction.
In accordance with the present invention, an apparatus for treating obstructive sleep apnea is provided, comprising a source of air and means for directing an air flow from the source to a patient. This part of the system may be of the type disclosed, for example, in U.S. Pat. No. 5,065,756, the entire contents of which being hereby incorporated by reference.
Rapoport, in U.S. Pat. No. 5,065,756, discloses a source of air and means for directing an air flow from the source to a patient. As shown in FIG. 2, a mask 40 suitable for fitting over the nose of the patients, includes a nose piece 10 and rim 11 for sealing the mask to the face. Thus, the air cuff seal 11 is made of a lightweight plastic material and must be non-irritating since it is in continuous contact with the patient""s face. The nosepiece 10 is made of a partially rigid and partially flexible material, such as heavy vinyl, of a nature that can conform to the face of the patient. The element must be sufficiently large to accommodate the noses of all patients who may employ the mask. The partial rigidity is required so that the nosepiece will generally maintain its shape in use, while still enabling it to conform to the face of the patient.
A harness 42 maintains the mask in position on the patient when the apparatus is used. In this arrangement, the mask 40 is connected directly to a compressor or blower 44 by a hollow flexible tube 46. An adjustable relief valve 48 is connected between the blower 44 and the mask 40 at a T fitting 50 inserted into the tube 46. The valve is mounted by any convenient conventional means at a location separate from the patient and mask, the flexible tube being sufficiently long that fixed mounting of the valve has no effect on the patient""s movements.
As illustrated in FIG. 3, the valve may be simply comprised of a rigid valve disk 51 held adjacent a valve seat 62 formed on one end of the T fitting 50. The disk 30 may be loosely axially guided at its edge by an enlarged diameter end extension 63 on the end of the T fitting 50. The valve disk 61 is urged toward the valve seat 62 by a spring, such as a helical spring 64 extending through the T-fitting 50 to a fixed connection, for example, to a pin 65 held to the walls of the T fitting. Adjustability of the pressure maintained by the valve may be affected la connecting the end of the spring 64 to the end of an adjustment screw 66 threaded in the disk 61. The adjustment of the screw thereby controls the tension of the spring, to determine the pressure of air directed to the mask. The valve is settable to enable the production of an operating pressure range within the mask of from about 5.0 to about 15.0 centimeters H2O. The pressure adjustment for any patient is set so that under normal breathing conditions the valve is always open, even during inhalation. As a result, the required positive pressure is always present to maintain the nasopharyngeal airway opened.
It is of course apparent that the illustrated valve constitutes only the preferred embodiment thereof, and that other constructions thereof for serving this function may alternatively be used in accordance with the present invention. The valve 48 continually discharges gases to the external atmosphere as indicated by the arrows 70 when the blower 44 provides a positive pressure in the system. The valve 46 is suitable to maintain a positive pressure within the system of about 5 to about 15 centimeters of water, with a discharge of air flow from the valve 48 in the range of 30 to 50 liters per minute.
A reservoir bag 54 connected to the flexible tube 46 between the valve 48 and the blower 44 serves to reduce transience in the flow rate and pressure within the system.
As illustrated in FIGS. 2 and 4, the mask 40 includes ports, preferably two ports 56 passing through the shaped portion 10 of the mask 40. Through these ports 56 air from the system, and particularly air exhaled by the patient, passes from the system to the external ambient environment. These ports 56 constitute intentional leaks at the mask, and must be small enough not to vent off all the pressure delivered by the compressor 44/valve 48 combination, but must be large enough to vent the patient""s expired breath over the period of expiration. For example, holes which are individually capable of passing a flow of 5 to 7 liters of air per minute with an internal mask pressure of 5 centimeters of water, and which have a diameter on the order of {fraction (1/16)} inch thick, have been found satisfactory. Suitable means for blocking one or both of these ports, such as plugs 57, may be provided in order to enable adjustment of the rate of air discharge form the mask. It is of course apparent that the invention is not limited to this size and number of ports.
When the pressure within the mask is at the low end of the operating range, that is, in the range of about 5-7 centimeters of water, at least two ports 56 with sizing as described above are left open to vent the mask at a rate of approximately 10-12 liters per minute. When the pressure within the mask is sent in the upper end of the range, form about 10-15 centimeters of water, alone hole is plugged, while the other provides a vent which delivers on the order of 5-7 liters per minute.
Since the valve 48 is not mounted on the mask, but is coupled thereto by a flexible tube, the weight of the mask assembly that must be supported on a patient""s face is substantially reduced, and the comfort to the patient is accordingly greatly increased. The connected between the tube 46 and the mask 40 may be via a swivel joint 58, if desired, to permit the patient to have more freedom of movement without the danger of entangling the mask apparatus with the bedding or causing the mask to separate from the face.
The compressed air may be provided by any conventional device, so that the patient may inexpensively provide this source for use in his own home. It is preferred, however, that a blower be provided instead of a compressor, since compressors tend to desiccate the air supply, while blowers deliver air at room humidity, can handle ultrasonically humidified air, and drop flow upon increases in back pressure. This latter feature is desirable, since the flow from a blower quickly increases during inspiration, when the back pressure increases in the system. The compressed air may be heated and humidified by conventional devices.
In addition, apparatus is provided for sensing the waveform of the airflow, to detect deviations therein that correspond to flow limitation in the air supplied to the patient, as well as apparatus for detecting cardiogenic oscillations to detect whether or not there is obstruction when the waveform detected is ambiguous. The deviations detected include deviations from a substantially sinusoidal waveform, flattening, or the presence of plateaus in the portions of the waveform corresponding to inspiration of the patient. In response to such variations in the airflow, the system of the invention increases or decreases the pressure to the patient in a known manner.
The method of the present invention provides for increasing the controlled positive pressure to the patient in response to the detection of flow waveform portions corresponding to flow limitations in the patient airway in response to variations in the airflow combined with the presence or absence of cardiogenic oscillations. The pressure increases may be effected periodically. Similarly, the controlled positive pressure may be periodically decreased in the absence of flow limitation or obstruction. The system may be provided with a program that periodically decreases the controlled positive pressure in the absence of detection of flow limitation or obstruction in the patient airway, and that periodically increase the pressure in the presence of detection of flow limitations classified as obstructions.
The first step in determining whether to increase or decrease the controlled positive pressure is to detect the presence of a valid breath and store an inspiratory waveform of that breath for further analysis. Next, the waveform of the stored breath is analyzed regarding its shape for presence of flow limitation. Whether flow limitation is present is in part determined by flow limitation parameters calculated from the shape of the waveforms of the current breath and of the immediately preceding breath. Once the presence of flow limitation has been analyzed, the system determines what action to take for adjustment of the controlled positive pressure. Where the waveform is ambiguous, the system determines if there is cardiogenic oscillation. If cardiogenic oscillation is present, there is no obstruction and no measures need to taken to overcome this obstruction. However, if cardiogenic oscillation is not present, there is obstruction, and the system is programmed to take appropriate action to overcome the effects of the obstruction.
An example of a breathing device or apparatus consists of a flow generator, such as a variable-speed blower, a flow sensor, an analog to digital converter, a microprocessor, and a pressure controller, such as a blower motor speed control circuit, a patient connection hose, a nasal coupling, and, optionally, a pressure transducer. Alternative patient circuits may be used, such as those disclosed in U.S. Pat. No. 4,655,213 and U.S. Pat. No. 5,065,756, the entire contents of which being hereby incorporated by reference.
One alternative patient circuit as shown in Rapoport, U.S. Pat. No. 4,655,231, includes a nose mask incorporating a threshold valve, wherein the air pressure continually applied to the mask is continually released form the mask, by means of a valve, at such a pressure that normally some pressurized air always escapes from the mask by way of the valve, at such a pressure that normally some pressurized air always escapes from the mask by way of the valve. This maintains the air pressure at the nose, in order to maintain the nasopharyngeal airway open, as well as to provide a continuous flow of fresh air to the mask so that the patient may exhale through the mask, with the exhaled air being immediately exhausted through the valve.
In one embodiment of the present invention, the blower supplies air through the flow sensor to the patient via a hose and nasal coupling. The microprocessor obtains the flow waveform from the digitized output of the flow sensor. The microprocessor then adjusts the speed of the blower via the motor control circuit to change the air pressure in the patient supply hose. A pressure transducer may be provided to measure the actual pressure in the patient hose. The microprocessor may store measured pressure and flow waveform values in its data memory to provide a history for real-time or off-line processing and analysis. When the waveform is ambiguous, the pressure or absence of cardiogenic oscillations is determined and appropriate action for the patient is taken.